Make clang happy with vdev_raidz_ code

Make clang happy with vdev_raidz_ code

The macros are used to generate code for conditions without a
corresponding branch. This is not a problem in practice, but
clang has no way of knowing that. Add a default branch with a
VERIFY(0) to indicate that it "can't happen"

```
In file included from \
/usr/home/mmacy/devel/ZoF/module/zfs/vdev_raidz_math_sse2.c:607:
/usr/home/mmacy/devel/ZoF/module/zfs/vdev_raidz_math_impl.h:281:3: \
error: no case matching constant switch condition '3' [-Werror]
```

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9434